A REVISION OF THE STRA.TIGRAPIDC IDSTORY OF THE GENUS STEPHANOLITHION
M. Kevin E. Cooper, E.C.L. Stratigraphic Services Ltd., Guildford, UK
INTRODUCTIONThe genus Stephanolithion is elliptical in shape with a modified protolith rim possessing lateralspines or protuberances. The central area has two to eight arms arranged, typically,
symmetrically around a small central vertical spine. The genus was first designated byDeflandre in 1939 and is one of the oldest establish nannofossils genera. Because of theirmorphology species are easily identified and have an uncluttered taxonomy and synonymy. This
has also led them being widely used as biostratigraphic marker species. One or more speciesof this genus have appeared in all the published zonation schemes for the Jurassic. In the mostrecent scheme (Bown, Cooper and Lord 1988) a total of six species of this genus were used indefining zones or subzones.
As a result of new species being defined and the adjustment of ranges of establishedspecies, it has become necessary to revise the stratigraphic history and evolutionary lineage ofthe genus Stephanolithion.
This paper is a result of research carried out to establish an early to mid Mesozoic
biozonation {Bown et al. 1988) and also as part of the author's Ph.D studies.
IDSTORY OF STUDY
The first detailed study of the genus Stephanolithion was undertaken by Rood and Barnard(1972). Even though they examined most of the Jurassic their study was selective andincomplete with some ammonite zones remaining unexamined. In that paper, the subspecies
S. speciosum octum and the species S.hexum were described. The study explained the evolutionof S. speciosum into S. speciosum octum and S. hexum but did not suggest any evolutionary link
to S. bigotii. Additionally, although showing Stephanolithion as ·continuing into the Early
Cretaceous with the species S. laffittei, the authors questioned the validity of the genericassignment of this species.
The next publication involving this genus was Medd (1979) on the Haddenham andGamlingay boreholes in Cambridgeshire. Medd mainly covered the Late Jurassic and recognised
one new species and two new subspecies; Stephanolithion carinatum, S. speciosum elongatumand S. bigotii maximum. No discussion of the evolutionary history of the genus was attempted
although the range of S. bigotii was extended upward into the Kimmeridgian.
Perch-Nielsen (1985), updated the evolutionary history of Stephanolithion to include thenew data from Medd (1979). Furthermore, she suggested that the ancestor of Stephanolithion
was Stradnerlithus comptus, which may have given rise to S.speciosum in the Early Bajocian.
She also considered that S.speciosum was the ancestor of S. speciosum octum and possibly S.
hexum, as well as Medd's S. speciosum elongatum and S. carinatum. She placed the Cretaceous
species S. laffittei into a separate genus Rotelapillus, thus restricting the genus Stephanolithion
to the Jurassic.
Cooper (1987) studied material from the type Volgian and from these samples one newspecies of Stephanolithion was described, S. atmetros, and a new combination was proposed; S.
helotatus (basionym Corollithion helotatus Wise & Wind 1977). Additionally, the last occurrenceof S. bigotii was extended from the Kimmeridgian up to the Middle Volgian.
EVOLUTIONARY LINEAGE
The first occurrence of the oldest species in this genus, S. speciosum, is considered to be at thetop of the Early Bajocian (see Fig.) The most likely ancestor for this species is Stradnerlithus
asymmetricus (Rood et al. 1971), Medd (1979). This species occurs in the same samples as S.
speciosum and is morphologically similar, having eight bars in the central area but lacking thelateral spines of S. speciosum. In the Late Bajocian, S. speciosum gave rise to S. speciosum
octum by an increase in the length of the lateral spines. The next evolutionary change occurredin the Bathonian with the first occurrence of S. hexum; this species has only six bars in thecentral area and long lateral spines. The probable ancestor for this species is S. speciosum
INA Newsletter 12/2 - 199017

octum which, although it has eight bars, does possess long lateral spines.
S. speciosum speciosum also gave rise to a number of other forms during the later part of
the Middle Jurassic. At the top of the Bathonian, S. elongatum first appeared. This species
is similar to S. speciosum speciosum in that it has short lateral spines and eight arms in thecentral area. However, its shape is considerably more elliptical than S. speciosum speciosum.
Also first recorded at this level is a large form of S. speciosum speciosuni very similar in size
to the forms of S.bigotii seen in the Early Oxfordian that have been assigned to the subspecies
S. bigotii maximum. Due to a lack of adequate photographs this form has not yet been formallydescribed. Another species with S. speciosum speciosum as its possible ancestor is S. carinatum
which appeared first in the Oxfordian. This latter species has short lateral spines and sevenor eight arms randomly arranged in the central area, in contrast to S. speciosum speciosum
which has symmetrically arranged arms.
The next significant event in the evolutionary history of the genus Stephanolithion was thefirst occurrence of S. bigotii bigotii in the lower part of the Callovian. This species appears tohave evolved from S. hexum by the loss of a pair of central area arms.
Subsequent
subspeciation appears to be evidenced by the occurrence of the short-ranging S. bigotii maximum(topmost Callovian to early Oxfordian). This subspecies is very similar in form to S. bigotii
bigotii but is much larger (8. bigotii bigotii has an average size with spines of 4.3 X 3.5 m
whereas S. bigotii maximum is defined as being over 6.0 X 3.0 m). Also probably related toS. bigotii bigotii is S. helotatus. It has four arms in the central area but its lateral spines arevery short, and can be absent on etched specimens. S. helotatus first appears in theKimmeridgian and ranges up into the Middle Volgian. In the Middle Volgian, the last true
species of Stephanolithion, S. atmetros appears. It has an elliptical shape, small or no lateralspines and a single pair of arms in the central area forming a crossbar parallel to the small
axis of the ellipse. It is likely that S. atmetros evolved from S. helotatus by the loss of, ormerging of, a pair of arms. The highest occurrence of this species in the Jurassic is basal
Portlandian and no true species of Stephanolithion are known from the Cretaceous.
The Cretaceous species S. laffittei, is now regarded as belonging to the genus Rotelapillus,
not Stephanolithion, and this species can be shown to have evolved from R. radians during theterminal Jurassic stage. The species R. radians, the ancestral form of the Rotelapillus, probably
evolved from the species Stradnerlithus fragilis an eight armed form of this genus. Both ofthese species have been recorded from the Middle Volgian and, even though S. fragilis iselliptical rather than round like R. radians, in these Middle Volgian samples, many of thespecimens of S. fragilis have very low elipticity.

ACKNOWLEDGEMENTS
I would like to acknowledge the research support of E.C.L. Stratigraphic Services in particularDr. J. Weston for grammatical advice.

SPECIES CITED
Rotelapillus laffittei (Noel, 1957) Noel 1973
Rotelapillus radians Noel 1973
Stephanolithion atmetros Cooper 1987
Stephanolithion bigotii Deflandre 1939
Stephanolithion bigotii maximum Medd 1979
Stephanolithion carinatum Medd 1979
Stephanolithion helotatus (Wind & Wise, 1977) Cooper 1987
Stephanolithion hexum Rood & Barnard 1972
Stephanolithion speciosum Deflandre 1954
Stephanolithion speciosum elongatum Medd 1979
Stephanolithion speciosum octum Rood & Barnard 1972
Stradnerlithus asymmetricus (Rood et al. 1971) Medd 1979
Stradnerlithus fragilis (Rood & Barnard 1972) Perch-Nielsen 198418

INA Newsletter 1212 - 1990

REFERENCES CITED
Bown P.R., Cooper M.KE. & Lord A.R. 1988: A Calcareous Nannofossil Biozonation Scheme forthe early to mid Mesozoic. Newsl. Stratigr., 20/2, 91-142.
Cooper M.K.E. 1987a: Calcareous nannofossils across the Jurassic/Cretaceous boundary.
Unpublished PhD Thesis, University College London ..
Cooper M.KE. 1987b: New calcareous nannofossil taxa from the Volgian Stage (Upper Jurassic)
lectostratotype site at QQrodishche, U.S.S.R.. N. Jb. Geol. Palaont. Mh., 1987(10), 606612.
Medd A.W. 1979: The Upper Jurassic coccoliths from the Haddenham and Gamlirtgay boreholes.(Cambridgeshire, England). Eclog. geol. Helv., 72, 19-109.
Perch-Nielsen K. 1985: Mesozoic calcareous nannofossils. In, H.M. Bolli, J.B. Saunders & K.
Perch-Nielsen (Eds.), Plankton Stratigraphy, Cambridge University Press, Cambridge, 329426.
Rood A.P. & Barnard T. 1972: On Jurassic coccoliths: Stephanolithion, Diadozygus and related
genera. Eclog. geol. Helv., 65, 327-342.
Wise S. & Wind F.H. 1977: Mesozoic and Cenozoic calcareous nannofossils recovered by D.S.D.P.
Leg 36 drilling on the Falkland Plateau, Southwest Atlantic sector of the Southern Ocean.
l.R.D.S.D.P., 36, 269-309.

INA Newsletter 12/2 - 1990

19

:;:oc...
tO

\I

11I

I
11Ill
11 II
1111
\c;_:::zIzCl)....o.zLower Middle Upper

5- ·a~·o1c.>

C" c;_i~z--~-c...

m......,::'?!

I\___,IIIIIII093 . . .

£-o~~rgo-QO(')Q:Im)>C>

Stephanolithion carinatum

Stephanolithion speciosum elongatumo@~~ Stephanolithion speciosum speciosum~ Stradnerlithus asymmetricus. ...,~

I.....,~()le;_Portlandian• _ - -G

Stephanolithion atmetros

Stephano/ilhion helotalus

L~I·~

Stephanolithion bigotii bigotii

Stephanolithion bigotii maximum

Stephanofilhion hexum~ Stephanolfthion speciosum octum~ Stephanolfthion sp.2 ('maximum?

i:~1\)c;_1

Tithonlan
IBerria1 · Upperslan L
VolgianLower I Middle iUpper
Lower

Lo~er 1 Upper.§:ill

2S3

TI = = mZ ImZ =z.

~~

IIII_zz....o.z

I Upper Lower IMiddlej Upper~·:s;rif&..,i;2g·_g!

;I t~Lower~

1..

LATE JURASSIC

I(")~-as-.

c: ::J....
s:u~ ::3c..{g;J'C/)_meo_::J (Q;J':::::!.-

c: 0o.3.

CD CD~3<D-o::;· 00CD<-CDtg'·Z~~!!lMIDDLE JURASSIC[!!.

~:z:~0t..:l